Hmm...
I think it's the latter one. But I seriously don't know for sure since it's your creation. But I guess since it's a published work you could call it just "(original) cosplay" and just mention in the information that it's actually your own creation since you wrote the source said character appears in.
Good luck!

I was thinking of cosplaying my own main character when my first manga volume comes out and I do not see that as an original cosplay as it is actually based on something other than just a thought.
I think you should go for it!